1. Field of the Invention:
This invention relates to a rotating magnetic head used in a magnetic recording reproduction system of videotape recorders.
2. Description of the Background Art:
Conventional methods for magnetic recording and reproduction includes automatic scan tracking (AST) and dynamic track following (DTF) and both are used for 1 inch VTR for broadcasting services. In both methods, the head tracks are moved in the width direction of the track in order to reproduce the information on the tracks of a magnetic tape.
FIG. 21 shows a block diagram of conventional rotating magnetic head for a magnetic recording reproduction system. The reproduction system in the diagram has fixed heads 1 and 2, movable heads 3 and 4, bimorph type piezoelectric elements 5 and 6, and a rotating drum 7.
The rotating drum 7 rotates at the speed of 180 rpm. On the periphery of the drum, the fixed heads 1 and 2, and the bimorph type piezoelectric elements 5 and 6 are attached with the same angle distance. At the top of the bimorph type piezoelectric elements 5 and 6, the movable heads 3 and 4 are attached. With this configuration, rotation of the rotating drum 7 causes the rotation of the fixed heads 1 and 2 and the movable heads 3 and 4. When a magnetic tape is rolled round the rotating drum 7, the heads 1 through 4 are rubbed against the magnetic tape one by one. The fixed heads 1 and 2 are usually used for normal reproduction. The movable heads 3 and 4 are used for special reproduction, when the AST or DTF technique is utilized.
FIG. 22 shows actions of the movable heads 3 and 4 at special reproduction. In this figure, the cross-sectional view of the neighborhood of the joint of the rotating drum 7 and the bimorph type piezoelectric element 5 is shown. In contrast with FIG. 21, the bimorph type piezoelectric element 5 and the movable head 3 is placed below the rotating drum 7.
A fixed drum 8 is on the same axis as the rotating drum 7 and is fixed to the covering box of a VTR. A magnetic tape 9 is rolled round the rotating drum 7 and the fixed drum 8 so that one of the fixed heads (1 and 2) or the movable heads (3 and 4) contacts the magnetic tape 9. In FIG. 22, the movable head 3 contacts with the magnetic tape.
The movable head 3 makes contact with a different area of the magnetic tape 9 by bending the bimorph type piezoelectric element 5 as shown by the dotted arrow in FIG. 22. The other movable head 4 has the same structure and performs the same actions.
The bimorph type piezoelectric element 5 bends in vertical directions in FIG. 22 according to a supplied voltage. FIG. 23 is a conceptual diagram of the mechanism. The bimorph type piezoelectric element 5 has piezoelectric elements 5a and 5b with three electrodes 5c, 5d and 5e between the two piezoelectric elements 5a and 5b. In other words, piezoelectric elements and the electrodes are laminated in the following order: 5c, 5a, 5d, 5b and 5e. One endpoint of the bimorph type piezoelectric elements 5 is fixed as described above, and the other endpoint of the bimorph type piezoelectric elements 5 is free and supports the movable head 3.
When alternating current voltage is added to the bimorph type piezoelectric element 5, the free endpoint oscillates due to expansion and contraction of piezoelectric elements 5a and 5b. As shown by arrows inside the piezoelectric elements 5a and 5b in FIG. 23, the piezoelectric elements 5a and 5b are polarized and the direction of polarization (direction of the arrow in the figure) is the same within a piezoelectric element. Moreover, the piezoelectric elements 5a and 5b are polarized and laminated so that the directions of polarization of the two piezoelectric elements coincide with each other.
When alternating current voltage in one direction is added between the electrodes 5d and 5c and voltage in the other direction is added between the electrodes 5d and 5e, one of the piezoelectric elements 5a and 5b expands while the other contracts as shown by the horizontal arrows in the FIG. 23. The expansion and contraction causes bending of the free endpoint the bimorph type piezoelectric element 5 to the contracting piezoelectric element, which results in the shifting of contact area of the movable head 5 and the magnetic tape 9.
FIG. 24 shows track patterns normally recorded on the magnetic tape 9. At normal recording, the recording is performed by fixed heads 1 and 2. The magnetic tape 9 is rolled around and covers approximately 190 degrees of the movable drum 7 and the fixed drum 8. Therefore, the fixed head 1 and the fixed head 2 alternately perform recording. In FIG. 24, A1, A2, . . . are the tracks recorded by the fixed head 1, and B1, B2, . . . are the tracks recorded by the fixed head 2 that has different azimuth angle from the fixed head 1.
At normal reproduction, the fixed heads 1 and 2 scan the tracks A1, B1, A2, B2 . . . in the order and reproduction is performed. The azimuth angle of the head at recording and the azimuth angle of the head at reproduction must be equal.
In FIG. 24, a control track and a linear audio track are indicated by C and D respectively.
Description of a mechanism for speedy searching reproduction of normal recording follows. In the following description, an example of speedy searching reproduction in which the searching speed is five times faster than normal reproduction is used.
Speedy searching is defined as a reproduction in which the information recorded on the magnetic tape 9 is reproduced at higher speed, usually an integer (2, 3, 4 . . . ) times faster, preferably without any noise.
In speedy searching five times faster than normal reproduction, if the voltage is not applied to the bimorph type piezoelectric elements 5 and 6 and the contact area of the movable head 3 to the magnetic tape is fixed, the scanning track of movable head 3 spans, for example, from A1 to A3. Among tracks between A1 and A3, the tracks to be reproduced by the movable head 3 are those tracks recorded with the same azimuth angle as the movable head. If the azimuth angle of the A1, A2, . . . are the same as the azimuth angle of the movable head 3, among A1, B1, A2, B2 and A3 the reproducible tracks are A1, A2, and A3. Therefore, during the scanning of the tracks B1 and B2, the movable head 3 generates noise and noise bars will be displayed on a reproduction screen.
In order to remove the noise in speedy searching, alternating current voltage is added to the bimorph type piezoelectric elements 5 and 6 supporting the movable heads 3 and 4. The alternating current voltage, called driving voltage of the movable head 3 and 4, makes the movable head 3 scan L2, which is identical to the track A1, and not L1. Specifically, it is a saw-tooth-wave like voltage that deviates the contact area to the magnetic tape of the movable head 3 or 4. The amount of deviation should be four track pitches per field period.
By use of the alternating current voltage the noise in the speedy searching disappears. The movable heads 3 and 4 scan the tracks A1, B3, A6, . . . , and noiseless speedy searching is enabled. If the driving voltage of the movable heads 3 and 4 is adjusted to the desired speed, speedy searching at any speed is possible in principle.
In practice, however, a rotating magnetic head having the same structure can not perform noiseless speedy searching if the speed is more than five times faster. This is because the movable heads 3 and 4 can not keep facial contact with the magnetic tape 9. The movable heads 3 and 4 have gaps and precise contact of the neighborhood of the gap to the magnetic tape is required for noiseless reproduction. When the width of the deviation of the movable head 3 and 4 is large, facial contact is disabled, and in more than five time speedy searching reproduction, the noise can not be removed.
On the other hand, in order to simplify the structure of the rotating magnetic head, decreasing the number of heads is quite effective. From this view point, if the movable heads can also perform normal recording/reproduction, the fixed heads can be abandoned.
By simply abandoning the fixed heads, however, good normal recording/reproduction can not be realized. Firstly, when reproducing information recorded by other instruments, the tip of the movable head sometimes has to make oblique contact with the magnetic tape. The contact angle of the movable heads toward the magnetic tape is reflected by C/N of the reproduction signal and oblique contact is a factor of poor C/N. Moreover, in normal recording with the movable heads, the reproduction FM signal can not be detected and the positioning control of the movable heads is very difficult. Fixing a movable head at a designated position is also difficult. These factors make recording in standard format extremely difficult.
The rotating magnetic head in which speedy searching reproduction is performed by positioning control of movable heads has the following problems: (1) more than five times faster reproduction than normal reproduction is impossible; and (2) normal recording/reproduction can not be properly realized.
A type of VTR, for example the one known as VHS-HiFi, employs multi-layer recording that records voices and pictures on different layers. FIG. 25 shows an outline structure of a rotating magnetic head, especially the positioning of heads, applied for VHS-HiFi VTR. The instrument records the HiFi audio signals (voices) on the deep layer of the magnetic tape, and the video signals (pictures) on the surface layer. To enable the above recording, the instrument includes six heads.
Of six heads, heads 11 and 12 are for deep layer recording/reproduction. The heads 11 and 12 have different azimuth angle and are placed in an axial symmetric position on the periphery of a rotating drum 17.
The video heads 20 and 21 are for surface layer recording. The azimuth angle of the heads 20 and 21 are also different and placed on the periphery 60 degrees away counterclockwise from the head 11 or 12. In the figure, the direction of magnetic tape movement is indicated by an arrow A and the tape is rolled round and covers 190 degrees of the periphery of the rotating drum 17.
13 and 14 are movable heads driven based on the same principle as the prior example. The movable heads 13 and 14 are placed 60 degrees away counterclockwise from the head 20 or 21.
FIG. 26 shows the principle of multi-layer recording in accordance with the conventional example.
The heads 11 and 12 have larger gaps than the heads 20 and 21. In this example, the gap width of the heads 11 and 12 is 0.9 mm, on the other hand the gap width of the heads 20 and 21 is 0.3 mm. The width of the heads 11 and 12 are 26 microns and the width of the heads 20 and 21 is 48 microns. There is a difference in height (the distance from the center of the rotating drum 17 to the tip of a head) between the heads 20 or 21 and the heads 11 or 12, and the heads 20 and 21 are 16 micro m higher than the heads 11 and 12. The recording current supplied to the heads 11 and 12 is stronger than the current supplied to the heads 20 and 21. The heads 11 and 12 perform recording before the heads 20 and 21.
A description of the recording follows by showing the actions taken by the heads 11 and 20.
As shown in FIG. 26, the magnetic tape 19 is composed of base 19a with thickness of, for example, 16 microns, and magnetic substance layer 19b with thickness of, for example, 4 microns on top of the base. The head 11 is supplied with recording current and records voices into the deep layer of the magnetic substance layer. The head 20 is supplied with weaker recording current and records pictures on the surface layer of the magnetic substance layer 19b. The voices are recorded on the deep layer, and the pictures are recorded on the surface layer.
Because of the difference in height of the heads 11 and 20, and because of the width selection, the area scanned by the head 14 on the magnetic tape 19 completely covers the area recorded by the head 11. Therefore, the audio signals recorded by the head 11 are not exposed on the surface layer, which avoids decreasing of C/N at reproduction.
FIG. 27 shows track patterns recorded on the two layers of the magnetic tape 19. FIG. 28 shows detailed description of track patterns B3 and A4 in FIG. 27.
The tracks A1, A2 . . . in FIG. 27 are recorded by the heads 11 and 14, B1, B2 . . . are recorded by heads 12 and 13, and C is a control track.
The tracks A1, A2, . . . and B1, B2, . . . are divided into an edge area and a main area by switching points where switching of reproduction signals take place. In FIG. 28, B3 and A4 are main areas, x0, x1, y0 and y1 are switching points, X and Y indicate the length of the edge area, and V indicates the length of the main area.
The magnetic tape 19 is rolled round and covers 190 degrees of the movable drum 17 and one head performs recording/reproduction of 180 degrees of the 190 degrees, excluding the edge areas. In other words, two heads, for example the heads 11 and 12, alternately perform recording/reproduction of 180 degrees. The switching point is the point where the switching of the heads takes place.
The main area is the interval surrounded by the switching points and on this area the voices and the pictures are recorded as described above. The rest of the track (outside the main area) is named edge area. An edge area corresponds to five degrees of a rotating drum 17. With the edge area, the disappearance of reproduction signals in the neighborhood of the switching points is avoided by providing continuous reproduction signals in alternate use of two heads.
When the movable head 13 is fixed, its scanning trace will be the dotted line L4 shown in FIG. 27. The trace is the same as the trace made when the five time speedy scanning reproduction is performed (see FIG. 24). The movable head being fixed, it also generates noise and noise bars will be displayed on the reproduction screen, which was the case in the previous example.
By driving the movable heads 13 and 14, and following the track A1 with the head 13 or 14, the noise disappears. This can be achieved by driving the movable head 13 with the bimorph type piezoelectric elements 15, as illustrated in FIG. 29. The principle of the movement is the same as the one shown in FIGS. 22 and 23, and the driving voltage is also a saw-tooth-wave.
A noiseless reproduction signal can be obtained by following the tracks recorded by the heads 11, 12, 20, and 21 with the movable head 13 and 14. As shown in FIG. 30, the reproduction signal is continuous, because of the switching of the movable heads 13 and 14. As shown in FIG. 30, the reproduction signal is continuous, because of the switching of the movable heads 13 and 14 at switching points and the existence of the edge area of five degrees around the switching point.
An instrument having this kind of structure, however, has different problems from the previous example. The problems are the expensiveness due to increase in complexity of the instrument and generation of jitters caused by repeated head hammering on the tape.
The instrument includes three pairs of heads for voices, pictures and special reproduction (including speedy searching reproduction). As the number of the heads increases, the number of hammering of the heads on the magnetic tape also increases. More accurately, small vibrations of unused heads (for example, voice heads) affect actions of working heads (for examples, movable heads).